Semiconducting and conducting organic materials, both polymers and molecules, have successfully been included in a large range of electronic devices, e g electrochemical devices, for instance as dynamic colorants in smart windows and in polymer batteries. Reversible doping and de-doping involving mobile ions switches the material between different redox states.
Electrochromic materials exhibit colour changes or changes in optical density as a result of electrochemical reduction and/or oxidation reactions. An electrochromic material can either be present as a solid, or exist as molecular, neutral or ionic species in an electrolyte solution. These materials have been used for the creation of electrochromic cells, where the passage of electric charge causes colour changes in the materials. Electrochromic cells are used in electrochromic devices of different kinds, and two principal categories of these devices can be distinguished. The two categories differ from each other mainly in the arrangement of the elements of the electrochromic cell.
The first category of electrochromic devices utilises a sandwich construction, and is used in applications such as automobile windows, building windows, sunglasses, large billboards, mirrors with variable reflectance, sunroofs etc. In this type of electrochromic device, continuous layers of electrochromic material and electrolyte (as well as other layers of e g ion reservoir material) are confined between two electrodes that completely cover the layers of electrochromic material and electrolyte. For the electrochromic device to be of use, at least one of said electrodes has to be transparent to let light through the device. This requirement is met in the prior art through the use of electrode materials such as indium-doped tin oxide (ITO), tin dioxide or fluorine-doped tin dioxide. The electrochromic materials used in these applications vary, but are often based on heavy metal oxides such as WO3 or conducting polymers such as polyaniline or polypyrrole. The conducting, electrochromic polymer poly-(3,4-ethylendioxythiophene)(PEDOT) has attracted much study, and sandwich devices incorporating this polymer have been realised.
The second category of electrochromic devices aim at providing an electrically updateable display for realisation on a flexible support. U.S. Pat. No. 5,754,329 describes such a display, in which the electrodes of the electrochromic device are placed in one and the same plane, contacting a layer of electrochromic material for the generation of local colour effects at the interface between the electrochromic material and the electrodes. U.S. Pat. No. 5,877,888 represents a further development of this device, describing a two-sided display. However, the arrangement of the component layers of the electrochromic device is similar to that of the device of the U.S. Pat. No. 5,754,329 patent, considering that the electrodes on either side of the display support contact electrochromic material only, and the generation of electrochromic effects is confined to the area of the electrodes. The electrochromic materials that are used in these devices are described in detail in U.S. Pat. No. 5,812,300.
Problems with the pixel matrices in the displays of the prior art mentioned above include the fact that they are difficult and expensive to manufacture. In particular, no electrochemical pixel devices have been disclosed which are truly capable of being mass produced. Furthermore, the practical use of the pixel elements in the prior art devices has been hampered by their comparatively high power consumption as well as their short lifetimes. Also, materials used in prior art devices suffer from a lack of environmental friendliness, processability and economic production possibilities. There is therefore a need for new and improved pixel devices for incorporation in matrices that may be used in displays.
Actually, the lifetime problem is found to be widespread and similar problems are experienced in other types of electrochemical devices, such as electrochemical diodes and transistors. One critical factor for the lifetime of such devices is the performance of their electrochemically active elements, i.e. the element which is supposed to provide for redox reactions. There is therefore a general need for improved electrochemically active elements, not only for pixel devices but also for any other type of electrochemical device.